Key Role of the Cation in the Crystallization of Chiral Tris(Anilato)Metalate Magnetic Anions

A complete study of the role played by the usually considered "innocent" cation in the synthesis of chiral tris(anilato)metalate magnetic complexes is presented. This study is based on the rational synthesis of the family of compounds formulated as A(3)[M-III(C6O4X2)(3)] with A(+) = [PPh3Et](+), [PPh3Pr](+), [PBu4](+), [NHep(4)](+), and [PPh4](+); M-III = Cr-III, Fe-III and Ga-III; and [C6O4X2](2-) (X = Cl, Br, and NO2; [C6O4X2](2-) = dianion of the 3,6 disubstituted derivatives of 2,5-dihydroxy-1,4-benzoquinone, H4C6O4). We show and explain the unexpected key role played by the cations in isolating chiral or achiral crystals of these [M-III(C6O4X2)(3-) anions with D-3 point group symmetry. Thus, among the 18 new compounds prepared and characterized, the six obtained with the cation [PPh3Et](+), formulated as [PPh3Et](3)[M-III(C6O4X2)(3)](3-) [M/X = Cr/Cl (1), Fe/Cl (2), Ga/Cl (3), Cr/Br (4), Fe/Br (5), and Ga/Br (6)] are chiral and crystallize in the chiral and polar R3 space group. These mixtures of enantio-pure chiral crystals are obtained using a racemic mixture of the anions and cations. Interestingly, if we use the [M-III(C6O4X2)(3)](3-) (X = NO2,) anions or if we change the cation (A), then we obtain achiral salts presenting the same A(3) M-III(C6O4X2)(3)](3-) formula (X = Cl, Br, and NO2) and the same monomeric D3 structure for the anion. In these last cases the anions crystallize in the polar but not chiral P2cb space group (for X = NO2) or in the centrosymmetric (which is neither chiral nor polar) P (1) over bar space group for A(+) = [PPh3Pr](+). These salts include the following: [PPh3Et](3)[Fe(C6O4(NO2)(2))(3)] (7), [PPh3Pr](3)[M-III(C6O4X2)(3)] [M/X = Cr/Cl (8), Fe/Cl (9), Ga/Cl (10), Cr/Br (11), Ga/ Br (12), and Fe/NO2 (13)], [PBu4](3)[M-III(C6O4X2)(3)] [M/X = Cr/NO2 (14) and Fe/NO2 (15)], [NHep(4)](3)[M-III(C6O4X2)(3)] [M/X = Cr/Cl (16), Cr/Br (17)] and [PPh4](3)[Fe(C6O4(NO2)(2))(3)] (18). Here we report the synthesis of all these salts and the crystal structure of compounds 2, 5, 9, 10, 14, and 15, as well as the spectroscopic and powder X-ray characterization of all the members of this huge family of compounds. Compound 10 contains the unknown [Ga(C6O4X2)(3)](3-) anion, whereas compounds 14 and 15 are the two first complexes of the type [M(C6O4X2)(3)](3-) with X = NO2. The magnetic properties of all the salts containing paramagnetic Fe(III) and Cr(III) ions are also reported. All of them have been satisfactorily reproduced with a simple monomeric model including zero field splitting (ZFS) of the corresponding S = 5/2 Fe(III) and S = 3/2 Cr(III) metal ions. This series constitutes a huge library of valuable precursors for the synthesis of extended 1D, 2D, and 3D coordination polymers with tunable magnetic ordering, chirality, and even porosity.